1. Field of the Invention
The present invention generally relates to a safety braking system for slowing or stopping a vertically moving object, such as an elevator car, in an over speed condition. More particularly, the present invention relates to an elevator safety brake system for slowing or stopping an elevator car having a carbon-carbon composite friction surface.
2. Description of the Prior Art
A typical safety braking system is attached to an elevator car and comprises a pair of wedge shaped brake shoes having substantially flat frictional surfaces. The flat frictional surfaces are ordinarily positioned on opposite sides of the stem portion of a T shaped guide rail supported on an elevator hoistway wall. These wedge shaped brake shoes are activated by a governor mechanism which forces the wedge shaped brake shoes along an adjacent guide shoe assembly which in turn forces the frictional surfaces of the brake shoes to make contact with the guide rail to slow or stop the car.
In a typical safety braking system, the wedges may be loaded with up to approximately 56,000 lb (250,000 N) normal force by applying approximately 8000 psi over a 7 in.sup.2 surface (55,000 kPa.times.0.0045 m.sup.2)). Using cast iron frictional surfaces having a nominal coefficient of friction with respect to the guide rail at approximately 6 m/s of approximately 0.15, the 56,000 lb (250,000 N) force acting upon a wedge creates a frictional force of approximately (11,200 lb (50,000 N) on the frictional surface of the wedge. In a conventional elevator cab design using cast iron frictional surfaces, there are four frictional surfaces which generate a total potential stopping force of approximately 45,000 lb (200,000 N).
As very tall buildings are built, high speed, high load elevators (typically 4 to 8 m/s but up to 12.5 m/s) have become necessary to service the numerous floors in such buildings. Such elevators have a load rating of up to about 16,000 kg. The safety breaking requirements of such elevators have become increasingly demanding. It has been determined that conventional gray cast iron cannot operate as a consistent friction material at high speeds and loads required by such modern elevator systems due to breaking failures caused by excessive wear and a reduced coefficient of friction caused by high frictional heating. Accordingly, there is a need for elevator safety brake shoes made with alternative friction materials which provide low wear and consistent high friction to accommodate the high speeds and loads of elevators installed in very tall buildings.